THE CLIMATE OF EOCENE FOSSIL LAKE (GREEN RIVER FORMATION, WYOMING) AS DETERMINED FROM VERTICAL AND LATERAL FACIES TRENDS
The Green River Formation in Fossil Basin is informally divided into the lower, middle, and upper units, each representing a significantly different climatic phase of Fossil Lake. Siliciclastics, bioturbated micrites, and kerogen-poor calcimicrites containing fossil fish dominate the lower unit. The wet climate and high precipitation-evaporation ratio resulted in a fresh-water lake in an over-filled lake basin.
The middle unit is composed of kerogen-rich laminated-micrite and contains abundant fossil fish. Fossil Lake was relatively fresh during middle unit time, but experienced some periods of dryer climate as indicated by interbeds of dolomicrite.
The vertical trends in the dolomite-dominated upper unit are complicated by lateral facies trends that indicate a lateral salinity-gradient. The facies trends can be identified by calcite-dolomite ratios, stable-isotope ratios, TOC, evaporite content, and paleontologic variations within single time-synchronous beds. In some beds, laminated dolomicrites and evaporites grade shoreward into laminated and bioturbated calcimicrite containing fossil fish. Carbonate d18O values decrease shoreward. These features clearly demonstrate a shoreward freshening within Fossil Lake. This demonstrates that both the lake-center and shore facies must be studied in order to develop a complete climate model.